There have been developed various UV-curable aqueous coating compositions for plastics which contain no environmentally hazardous organic solvent. For example, German Patent No. 2936039 discloses a coating composition comprising a water-dispersible urethane acrylate prepared using amino sulphonic acid-alkali metal or—alkaline earth metal salt. However, this method provides a coating film having poor water resistance due to the alkali metal ion present on the film surface.
U.S. Pat. No. 4,287,039 discloses a method of dispersing a polyester urethane acrylate by using a dispersant. However, the film produced by this dispersion method shows poor water resistance, hardness, gloss due to the presence of moisture on the film.
U.S. Pat. Nos. 5,135,963, 6,011,078, 6,207,744, 6,335,397, 6,436,540, and 6,538,046 disclose curable polyurethane aqueous coating dispersions. The UV-curable polyurethane aqueous coating dispersions disclosed in these patents are obtained by using oligomers having larger molecular weights to ensure the stability of dispersion in water, but films obtained by using these dispersions have lower crosslinking densities, which does not fulfill plastic coating requirements.